The production of a cyclic diester from undecanedioic acid and ethylene glycol by depolymerization of the corresponding polyester composed of the acid and the glycol in the absence of a catalyst has been described by Carothers and Hill [J. Amer. Chem. Soc. 55, 5,031-5,039 (1933)]. In this process, cyclo(ethyleneundecanedioate) is obtained in trace amounts only. The cyclic dimer is formed as the primary reaction product.
The preparation of cyclic diesters by the thermolysis of linear polyesters in the presence of inorganic salts, such as the chlorides, nitrates, carbonates, or oxides of magnesium, manganese, iron, cobalt, or tin as the catalysts has been described in French Pat. No. 796,410. This reference contains no disclosure concerning the production of cyclic diesters of undecanedioic acid. If an attempt is made to produce cyclic diesters of undecanedioic acid according to this process, yields of merely less than 15% are attained.
Better yields can be achieved according to the processes described in Japanese Laid-Open Applications Nos. 4,826,790 and 4,920,503, wherein linear polyesters of aliphatic dicarboxylic acids and diols are treated at temperatures of 260.degree.-280.degree. C. under reduced pressure with lead nitrate and/or lead dioxide. Examples with undecanedioic acid are not disclosed in either reference. Both processes have the disadvantage that lead compounds must be used as the catalysts, the use of which is increasingly being restricted due to the notorious toxicity of lead compounds. Even worse, the cyclic diesters produced according to these methods themselves contain quite considerable quantities of lead. Since the cyclic diesters are utilized primarily in the cosmetics industry as fixing agents in perfumes, soaps, or mouthwashes (see DOS [German Unexamined Laid-Open Application] No. 2,440,526), the use of such lead compounds is prohibited. Even the elimination of the lead-containing residue from the thermolysis product is problematic because of the attendant environmental hazards due to, inter alia, lead-containing waste air emanating during the combustion of the residue, as well as poisoning of the wastewater and/or ground water if the residues are dumped.
A general disadvantage of all heretofore described processes for the production of cyclic diesters by the catalytic depolymerization of the corresponding polyesters resides in the rapid rise in viscosity which takes place in the reaction charge. This occurs even at the beginning of the splitting-off reaction undergone by the cyclic diester. It continues until crosslinking occurs, which makes it impossible to agitate the reaction batch after about only 10-15% of the cyclic diester has been split off, based on the amount of polyester employed, as can be seen from Comparative Examples 10-18 herein. For this reason, the heretofore described processes operate without any agitation of the thermolysis batch. This, however, is possible only when the polyester is thermolyzed on a laboratory scale, i.e., using only 50-100 g per batch. In order to depolymerize larger amounts of polyester in larger devices, it is absolutely necessary to agitate the reaction charge in order to provide sufficient heat transfer. Furthermore, the removal of the crosslinked residue of the thermolysis entails great difficulties in industrial batches. Thus, it is quite apparent that the aforementioned processes are entirely unsuitable for the production of the cyclic diesters in large quantities.
As a result, Japanese Application Sho-48/111 299 suggested a process for the production of cyclic esters by the depolymerization of linear polyesters under reduced pressure in the presence of dialkyl lead or tin oxides and an azeotropic agent. The azeotropic agent (paraffin oil), which is simultaneously also used to prevent a rise in melt viscosity during the thermolysis, is removed during the depolymerization together with the cyclic diester by azeotropic distillation. The cyclic diester is isolated from the resultant mixture by extraction and then purified by distillation.
It is clearly apparent that this process, as compared to the mere thermolysis, comprises several additional operating steps. Most significantly, difficulties are encountered in the purification of the cyclic ester because the azeotropic agent has a similar boiling point, and/or the procedure requires great technical expenditures.